Doctors have long warned women that getting pregnant later in life can raise the risk of stroke, but a study on Monday suggested that actually, only young women face this increasing risk.

The findings in the Journal of the American Medical Association (JAMA) Neurology compared stroke rates among pregnant and non-pregnant women of different age groups.

Previous studies have focused on the rate of stroke among pregnant women of different ages – finding stroke is more common among older women – but have not included a non-pregnant, aged-matched control group for comparison.

“Despite stroke being a rare event in young women, 18 per cent of all strokes in women younger than 35 years were associated with pregnancy,” said the study led by Eliza Miller of Columbia University.

In contrast, among older women of childbearing age, 1.4 per cent of strokes were associated with pregnancy.

The findings are based on data on women hospitalised due to strokes in the state of New York from 2003 to 2012. Of over 19,000 women admitted for strokes during that decade, just over four per cent were pregnant or had delivered a baby just six weeks before. Women aged 24 and younger had more than twice the risk of stroke in the pregnancy and six-week post-partum period (14 strokes per 1,00,000 women) as their non-pregnant counterparts (six strokes per 1,00,000 women).

Among women aged 25 to 34, pregnancy associated stroke happened at a rate of 21.2 per 1,00,000 pregnant women, compared to 13.5 per 1,00,000 non-pregnant women. Strokes became more common among women aged 35 to 44, but surprisingly, there was hardly any difference in stroke rate whether women were pregnant (33 per 1,00,000) or not (31 per 1,00,000). In the next age group, 45 to 55, stroke rates were far higher among non-pregnant women (74 per 1,00,000) than among pregnant women (47 per 1,00,000).

“Although older pregnant women had higher rates of stroke in pregnancy than younger pregnant women, their risk of stroke was similar to women of their own age, who were not pregnant,” said the study.

But in women below 35, pregnancy increased the risk of stroke, more than doubling it in the youngest group. More research is needed to understand why pregnancy raises the risk of stroke in young women, researchers said.

“Although older women have an increased risk of many pregnancy complications, a higher risk of stroke may not be one of them,” said the study.